In the viewpoint of miniaturization of transmission devices, reduction in operation costs, environmental issues, and the like, there has been a growing need for high-efficiency operation power amplifiers in recent years. When a power amplifier is operated in high efficiency, an input power is not linearly amplified in a region in which the input power is relatively high, and nonlinear distortion occurs therein. For this reason, there is a case in which an amplification method combined with digital predistortion in which inverse distortion to the nonlinear distortion that occurs in the power amplifier is added to a transmission signal in advance is used. The distortion added to a transmission signal in advance in the digital predistortion is also called distortion compensation coefficient, and is read from a lookup table, or calculated by using a polynomial expression. The distortion compensation coefficient is appropriately updated to adequately compensate the nonlinear distortion that varies according to an environment such as temperature.
On the other hand, to improve a capacity of a communication system by reducing interference of a transmission signal to unintended destinations, there is a case in which beamforming to form a directional beam is performed. When the beamforming is performed, phase differences are set to signals that are transmitted from plural antenna elements constituting an array antenna. To each of the antenna elements, for example, a phase shifter is provided to control a phase of a signal. The signal, the phase of which is controlled is amplified by a power amplifier corresponding to each antenna element.
It has been proposed to perform the digital predistortion on transmission signals collectively at one time also in the case in which a power amplifier is provided to each of plural antenna elements as described. That is, it has been considered that after a transmission signal is subjected to distortion compensation based on the distortion compensation coefficient, this transmission signal is split, and then amplified by the power amplifier of each antenna element, to be transmitted from each antenna element. In this case, update of the distortion compensation coefficient is performed, for example, by a method as follows.
A first method is to update the distortion compensation coefficient by using a feedback signal, providing, for example, a phase shifter to a feedback route from each antenna element. That is, an inverse weight to an antenna weight for the beamforming is added to a feedback signal from each antenna element by the phase shifter.
Moreover, a second method is to update the distortion compensation coefficient by using a feedback signal, providing an independent feedback circuit including an analog/digital (A/D) converter to each of the antenna elements. That is, an inverse weight to an antenna weight for the beamforming is added to a feedback signal from each antenna element by digital processing after A/D conversion.
By using these methods, a distortion compensation coefficient to compensate variations in amplitude and phase due to nonlinear distortion by removing an influence of phase variations caused by an antenna weight for the beamforming can be calculated.    Patent Document 1: Japanese Laid-open Patent Publication No. 2002-190712    Patent Document 2: Japanese Laid-open Patent Publication No. 2004-135263    Patent Document 3: Japanese Laid-open Patent Publication No. 2009-213113    Patent Document 4: Japanese Laid-open Patent Publication No. 2016-100835    Patent Document 5: International Publication Pamphlet No. WO 2016/167145    Non-Patent Document 1: Hamanaka, Toshiki, et al., “A Simple Nonlinear Compensation Method for Beamforming Transmitter”, Proceedings of the 2015 IEICE General Conference, 2015_Communication (1), p. 429, Feb. 24, 2015    Non-Patent Document 2: Park, Chan-Won, et al., “A new digital predistortion technique for analog beamforming systems”, IEICE Electronics Express, Vol. 13, No. 2, 1-7, Jan. 7, 2016
However, when the digital predistortion is performed in an array antenna structure, there is a disadvantage that a circuit scale of a feedback system increases. Specifically, for example, in the first method described above, because a phase shifter that adds an inverse weight to an antenna weight is provided to each antenna element, the circuit scale of the feedback system increases. Moreover, in the second method described above, because an independent feedback circuit that is equipped with an A/D converter and the like is provided to each antenna element, the circuit scale of the feedback system increases.
If the circuit scale of the feedback system increases as described, the size of the entire device increases and power consumption increases. Particularly, in recent years, it has been considered to arrange plural antenna elements not only linearly in one row, but also two-dimensionally on a plane, and the number of antenna elements tends to increase. With the increase of the antenna elements, there is a possibility that the circuit scale of the feedback system further increases.